Firefighting foam concentrates contain mixtures of surfactants that act as foaming agents, together with solvents and other additives that provide the desired mechanical and chemical properties for the foam. The concentrates are mixed with water in situ and foamed by mechanical means, and the resulting foam is projected onto the fire, typically onto the surface of a burning liquid. The concentrates are typically used at a concentration of about 1-6%.
Aqueous film-forming foam (AFFF) concentrates are designed to spread an aqueous film on the surface of hydrocarbon liquids, which increases the rate at which the fire can be extinguished. This spreading property is made possible by the use of perfluoroalkyl surfactants in AFFF, which produce very low surface tension values in solution (15-20 dynes cm−1), thereby permitting the aqueous solution to spread on the surface of the hydrocarbon liquids.
However, typical AFFF foams are not effective on fires caused by water-miscible fuels, such as low molecular weight alcohols, ketones, and esters and the like, because the miscibility of the solvent leads to dissolution and destruction of the foam by the fuel. To address this issue, alcohol resistant AFFF (AR-AFFF) concentrates are used, which contain a water-soluble polymer that precipitates on contact with a water-miscible fuel, creating a protective layer between the fuel and the foam. Typical water-soluble polymers used in AR-AFF are polysaccharides, such as xanthan gums. AR-AFFF foams are effective on both hydrocarbon and water-soluble fuels.
Conventional AFFF concentrates contain mixtures of perfluoroalkyl and non-fluorinated surfactants, each of which may be anionic, cationic, nonionic or amphoteric, solvents such as glycols and/or glycol ethers, and minor additives such as chelating agents, pH buffers, corrosion inhibitors and the like. Various AFFF concentrates are described in, for example, U.S. Pat. Nos. 3,047,619; 3,257,407; 3,258,423; 3,562,156; 3,621,059; 3,655,555; 3,661,776; 3,677,347; 3,759,981; 3,772,199; 3,789,265; 3,828,085; 3,839,425; 3,849,315; 3,941,708; 3,95,075; 3,957,657; 3,957,658; 3,963,776; 4,038,198; 4,042,522; 4,049,556; 4,060,132; 4,060,489; 4,069,158; 4,090,976; 4,099,574; 4,149,599; 4,203,850; and 4,209,407. AR-AFFF concentrates are described in, for example, U.S. Pat. No. 4,060,489; U.S. Pat. No. 4,149,599 and U.S. Pat. No. 4,387,032.
Until recently, aqueous film forming foams that were used for fire fighting invariably contained components, including low molecular weight fluorosurfactants and fluoropolymer surfactants having perfluoroalkyl chains where the perfluoroalkyl group was at least a perfluorooctyl (C8) group. For example, it was believed that a surfactant required at least a perfluorooctyl moiety to provide the necessary physicochemical attributes for efficient and persistent foam formation for fire fighting applications. See WO03/049813. However, perfluorooctyl moieties have been shown to be environmentally persistent and to accumulate in the livers of test animals, leading to calls for the phase-out of materials, including foam components, containing a perfluorooctyl group. Recent regulatory efforts such as the United States EPA's PFOA Stewardship Program and EC directives pertaining to telomer-based higher homologue perfluorinated surfactants have sought to discourage use of perfluorooctyl-containing components.
Fluorocarbon surfactant suppliers and formulators have been seeking to replace C8+ perfluoroalkyl chain products with C6 or lower perfluoroalkyl chain products to avoid the potential for environmental harm due to the persistence of the residual fluorinated alkyl chain. Perfluorohexanoic acid (PFHxA), the presumed ultimate decomposition product of C6 fluorocarbon surfactants has been shown to be non-toxic and non-bio-accumulative, however. The EPA and other regulatory authorities therefore prefer the use of C6 over C8 and higher perfluoroalkyl homologs and are actively pursuing regulation of the production and importation of such C8 and higher perfluoroalkyl products.
Although C6 fluorosurfactants have been reported to be satisfactory for less demanding applications, such as cleaning solutions, the reduction in length of the perfluoroalkyl chain unfortunately leads to a decrease in the ability to form long lasting persistent foams with the properties necessary for effective fire fighting. Thus, AFFF and AR-AFFF concentrates where the perfluorooctyl surfactant is replaced by an equivalent C6 compound typically are unable to meet the requirements of the US and international standards for fire fighting applications.
To counter this loss of activity, manufacturers have been forced to increase the concentration of fluorosurfactant in AFFF concentrates and/or to use oligomeric surfactants in which multiple perfluoroalkyl groups are covalently attached to short polymeric carrier molecules. See WO01/030873. In both instances the total concentration of fluorine atoms (calculated on a weight percentage basis) remains at an undesirably high level.